Analysis of a sample of biological origin frequently requires the separation of mixtures of biomolecules, such as nucleic acids, proteins, and polypeptides, which often have limited sample size. Electrophoresis, in which charged molecules move in a liquid under the influence of an electric field, has long been the method of choice for separating many classes of biomolecules. This method takes of advantage of differing migration velocities, vep, of different molecules.
The migration velocity, vep, is the distance (L) a molecule or particle moves per unit time (t). The migration velocity is the product of the electrophoretic mobility, μep, multiplied by the electric field strength E (units of volts/cm):vep=μep×E μep=q/6πηR where q is the charge on the particle and η is the viscosity of the medium and R is the radius of the particle. The velocity is, thus, directly proportional to the charge on the particle and the field strength and inversely proportional to the size of the particle and the viscosity of the medium. For relatively large particles or biomolecules, the charge increases as the size of the molecule increases, and the charge to mass (or radius) ratio becomes nearly constant. Under these circumstances if the electrophoresis is carried out in the presence of a gel composed of agarose (agarose gel electrophoresis) or crosslinked polyacrylamide (polyacrylamide gel electrophoresis), the gel structure creates a molecular sieving effect that allows the molecules or particles to be separated on the basis of size.
Capillary gel electrophoresis is typically carried out in 50 μm diameter capillaries that are 10 cm to 1 m long with a field strength that is generally in the range of 100 V/cm to 500 V/cm, and requires a high-applied voltage greater, typically greater than 1 KV. Heat generation is directly proportional to the square of the applied voltage, and the voltages required to achieve separation in capillary electrophoresis may cause degradation of sensitive samples.
A subject of a considerable amount of research in recent years has been microscale fluid handling systems that perform fast, automated, high-resolution sample preparation, reaction, and separation. Currently, this is being accomplished through advances in microfluidics. The idea is that once the manipulation of fluids can be mastered on the microscale, key experiments for biomolecule separation and analysis can be integrated and automated—all on a mass-produced chip. In microfluidic-based devices, nucleic acid molecules, proteins, polypeptides and other such molecules are transported, manipulated, and separated through miniature channels embedded into the chip. Detection systems can also be integrated into the chip or affixed externally as a separate component for seamless, automated and highly sensitive detection.